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Rice fields foster biodiversity

Restoration ecologists want to restore ecosystems that have been damaged or destroyed by human activity. One approach they use is “rewilding” – which can mean different things to different people. To some, rewilding involves returning large predators to an ecosystem, thereby reestablishing important ecological linkages. To others, rewilding requires corridors that link different wild areas, so animals can migrate from one area to another. One common thread in most concepts of rewilding is that once established, restored ecosystems should be self-sustaining, so that if ecosystems are left to their own devices, ecological linkages and biological diversity can return to pre-human-intervention levels, and remain at those levels in the future.

Chieko Koshida and Naoki Katayama argue that rewilding may not always increase biological diversity. In some cases, allowing ecosystems to return to their pre-human-intervention state can actually cause biological diversity to decline. Koshida and Katayama were surveying bird diversity in abandoned rice fields, and noticed that bird species distributions were different in long-abandoned rice fields in comparison to still-functioning rice fields. To follow up on their observations, they surveyed the literature, and found 172 studies that addressed how rice field abandonment in Japan affected species richness (number of species) or abundance. For the meta-analysis we will be discussing today, an eligible study needed to compare richness and/or abundance for at least two of three management states: (1) cultivated (tilled, flood irrigated, rice planted, and harvested every year), (2) fallow (tilled or mowed once every 1-3 years), and (3) long-abandoned (unmanaged for at least three years).

Three different rice field management states – cultivated, fallow and long-abandoned – showing differences in vegetation and water conditions. Credit: C. Koshida.

Meta-analyses are always challenging, because the data are collected by many researchers, and for a variety of purposes. For example, some researchers may only be interested in whether invasive species were present, or they may not be interested in how many individuals of a particular species were present. Ultimately 35 studies met Koshida and Katayama’s criteria for their meta-analysis (29 in Japanese and six in English).

Overall, abandoning or fallowing rice fields decreased species richness or abundance to 72% of the value of cultivated rice fields. As you might suspect, these effects were not uniform for different variables or comparisons. Not surprisingly, fish and amphibians declined sharply in abandoned rice fields – much more than other groups of organisms. Abundance declined more sharply in abandoned fields than did species richness. Several other trends also emerged. For example, complex landscapes such as yatsuda (forested valleys) and tanada (hilly terraces) were more affected than were simple landscapes. In addition, wetter abandoned fields were able to maintain biological diversity, while dryer abandoned fields declined in richness and abundance.

The effects of rice field abandonment or fallowing for eight different variables. Effect size is the ln (Mt/Mc), where Mt = mean species richness or abundance for the treatment, and Mc = mean species richness for the control. The treated field in all comparisons was the one that was abandoned for the longer time. A positive effect size means that species richness or abundance increased in the treated (longer abandoned) field, while a negative effect size means that species richness or abundance declined in the treated field. Numbers in parentheses are number of data sets used for comparisons.

When numerous variables are considered, researchers need to figure out which are most important. Koshida and Katayama used a statistical approach known as “random forest” to model the impact of different variables on the reduction in biological diversity following abandonment. This approach generates a variable – the percentage increase in mean square error (%increaseMSE) – which indicates the importance of each variable for the model (we won’t go into how this is done!). As the graph below shows, soil moisture was the most important variable, which tells us (along with the previous figure above) that abandoned fields that maintained high moisture levels also kept their biological diversity, while those that dried out lost out considerably. Management state was the second most important variable, as long-abandoned fields lost considerably more biological diversity than did fallow fields.

Unfortunately, only three studies had data on changes in biological diversity over the long-term. All three of these studies surveyed plant species richness over a 6 – 15 year period, so Koshida and Katayama combined them to explore whether plant species richness recovers following long-term rice field abandonment. Based on these studies, species richness continues to decline over the entire time period.

Plant species richness in relation to time since rice fields were abandoned (based on three studies).

Koshida and Katayama conclude that left to their own devices, some ecosystems, like rice fields, will actually decrease, rather than increase, in biological diversity. Rice fields are, however, special cases, because they provide alternatives to natural wetlands for many organisms dependent on aquatic/wetland environments (such as the frog below). In this sense, rice fields should be viewed as ecological refuges for these groups of organisms.

Rana porosa porosa (Tokyo Daruma Pond Frog). Credit: Y. G. Baba

These findings also have important management implications. For example, conservation ecologists can promote biological diversity in abandoned rice fields by mowing and flooding. In addition, managers should pay particular attention to abandoned rice fields with complex structure, as they are particularly good reservoirs of biological diversity, and are likely to lose species if allowed to dry out. Failure to attend to these issues could lead to local extinctions of specialist wetland species and of terrestrial species that live in grasslands surrounding rice fields. Lastly, restoration ecologists working on other types of ecosystems need to carefully consider the effects on biological diversity of allowing those ecosystems to return to their natural state without any human intervention.